Reconstructing East African rainfall and Indian Ocean sea surface temperatures over the last centuries using data assimilation

Abstract

The relationship between the East African rainfall and Indian Ocean sea-surface temperatures (SSTs) is well established. The potential interest of this covariance to improve reconstructions of both variables over the last centuries is examined here. This is achieved through an off-line method of data assimilation based on a particle filter, using hydroclimate-related records at four East African sites (Lake Naivasha, Lake Challa, Lake Malawi and Lake Masoko) and SSTs-related records at six oceanic sites spread over the Indian Ocean to constrain the Last Millennium Ensemble of simulations performed by CESM1. Skillful reconstructions of the Indian SSTs and East African rainfall can be obtained based on the assimilation of only one of these variables, when assimilating pseudo-proxy data deduced from the model CESM1. The skill of these reconstructions increases with the number of particles selected in the particle filter, although the improvement becomes modest beyond 99 particles. When considering a more realistic framework, the skill of the reconstructions is strongly deteriorated because of the model biases and the uncertainties of the real proxy-based reconstructions. However, it is still possible to obtain a skillful reconstruction of SSTs over most of the Indian Ocean only based on the assimilation of the six SST-related proxy records selected, as far as a local calibration is applied at all individual sites. This underlines once more the critical role of an adequate integration of the signal inferred from proxy records into the climate models for reconstructions based on data assimilation.

Keywords

Data assimilation East Africa Indian Ocean Model-data comparison

Electronic supplementary material

The online version of this article (doi:10.1007/s00382-017-3853-0) contains supplementary material, which is available to authorized users.

Notes

Acknowledgements

We thank the two anonymous reviewers for their careful reading and the constructive comments that helped improving the manuscript. This work is supported by the BRAIN-be programme of the Belgian Federal Science Policy Office (BelSPO) through project BR/121/A2 “Patterns and mechanisms of climate extremes in East Africa” (PAMEXEA). We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. Hugues Goosse is research director with the FRS/FNRS, Belgium.